The invention relates to an electric switching circuit for controlling a heater preferably for a flow of air in a laundry dryer
Laundry drying machines, also known as laundry dryers, have a fan for generating a flow of air and preferably also a heater for heating the flow of air in order to dry laundry. The fan is typically driven by an electric motor. With special operating statuses of the laundry dryer, e.g. with very wet laundry, with a generally high load and/or with high ambient temperatures, there is a risk of the electric motor overheating. In this case, the electric motor is typically automatically disconnected by means of an existing protective circuit. Disconnecting the electric motor nevertheless results in the fan driven by the electric motor then stopping and a flow of air no longer moving through the laundry dryer. In order in this case to prevent the flow of air heater overheating and causing damage to the heater itself, to a heater channel, to the wheel of the fan or to another component, it is usual in the prior art to provide a switch in series with the heater for the flow of air in order to be able to disconnect the heater in response to an actual operating state of the electric motor. It is thus particularly known to immediately disconnect the heater if a failure of the electric motor driving the fan is detected.
This controlled switch is traditionally realized in the form of a relay or centrifugal force switch for instance. The relay is typically connected such that it detects the absence of a current through the electric motor and thereupon opens the switch in series with the heater. The centrifugal force switch detects the absence of a rotation of the electric motor and thereupon switches the switch assigned to the heater open.
The centrifugal force switch is relatively expensive. Simple relays are certainly relatively cost-effective but often fail to meet prescribed safety standards. In contrast, the use of relays, which meet such safety standards, in other words which are designed for instance with materials which meet the stricter requirements placed on flammability and filament testing according to the IEC 60335.4 standard, particularly in the case of devices produced in series such as laundry dryers, represent a cost factor which is not to be ignored.